1. Field of the Invention
The present invention relates to a leading wave position detecting unit, method and the like for detecting the position of the leading wave in a delay profile.
2. Description of Related Art
FIG. 1A is a diagram illustrating a case where a mobile station receives a radio wave from a base station antenna along with radio waves that are reflected or diffracted from buildings and the like and arrive at the mobile station, and FIG. 1B is a diagram illustrating an example of a (propagation) delay profile. The radio waves (1), (2) and (3) in the delay profile are called an elementary wave (path).
In the delay profile as illustrated in FIG. 1B, the axis of abscissas represents the propagation delay time (called “delay time” from now on) of incoming radio waves to the mobile station, and the axis of ordinates represents the received power. The axis of ordinates can represent a propagation loss. The received power or the propagation loss can be represented in terms of an absolute value or a relative value.
FIG. 2 is a block diagram showing a configuration of a conventional receiver. The receiver as shown in FIG. 2 comprises an antenna 120, despreading code generators 122 and 136, multipliers 124 and 138, a delay profile measuring section 126, an averaging section 127, a noise level detector 128, an eliminator 130, a path selector 132, a detection timing setting section 134, and a RAKE combiner 140.
Receiving a signal via an antenna 120, the multiplier 124 multiplies it by a despreading code generated by the despreading code generator 122 so that the received signal is despread and the paths are divided. The delay profile measuring section 126 measures the received powers of the divided paths, thereby measuring (producing) the delay profile.
Since the measured delay profile includes noise, it is suppressed by the averaging section 127, noise level detector 128, and eliminator 130.
The path selector 132 selects the paths (paths with large received power, for example) appropriate for the RAKE combining from the noise-suppressed delay profile. The detection timing setting section 134 sets detection timing in consideration of the paths selected by the path selector 132. The despreading code generator 136 generates a despreading code in response to the detection timing determined by the detection timing setting section 134. The multiplier 138 despreads the signal received via the antenna 120, and the path selector 132 selects the paths to be supplied to the RAKE combiner 140 from the received signal. The RAKE combiner 140 carries out the RAKE combining of the input paths. The RAKE combined signal undergoes deinterleaving and the like thereafter. Thus, the finally demodulated data is obtained.
Assume that a communication unit A transmits a signal to a communication unit B, and the communication unit B measures the delay profile of the received signal. If the position of the leading wave can be detected in the measured delay profile, the distance between the communication unit A (transmitting site) and the communication unit B (receiving site) can be obtained.
In the example of FIGS. 1A and 1B, the leading wave is 20 the path (1).
The delay profile as illustrated in FIG. 1B, however, is an ideal delay profile, and when an actual delay profile is measured, the path position remains undeterminable because of reception (detection) timing.
This will be described taking an example of CDMA. The received signal is received in the form of a signal (c) of FIG. 3 consisting of a superposition of a signal (a) of FIG. 3 and a spreading code (b) of FIG. 3. The spreading code (short code) remains invariant for each symbol so that the delay profile is observed for each symbol. The observational length for each symbol is commonly called window width.
To obtain the delay profile by despreading the received signal (transmitted signal), waveforms (correlation outputs) as shown in FIGS. 4A and 4B are lo obtained in which the paths (A), (B) and (C) are repeated.
Thus, the path position remains undeterminable because of the reception (detection) timing.
For example, actual measurement of the delay profile as illustrated in FIG. 1B can provide path position relationships as shown in FIG. 5A, FIG. 5B or FIG. 1B, which means that the path alignment is uncertain. Therefore, it is desired to establish a method of detecting the path associated with the leading wave. Here, the order of path (1)→path (2)→path (3)→path (1) is fixed.
Although one symbol length is commonly much longer than a maximum delay time, if it is not much longer than the maximum delay time, the window width can be equivalently widened by combining the short code with a long code.
In addition, in the actual measurement of the delay profile, the waveforms blunt because of a frequency bandwidth limit. The delay profiles as illustrated in FIGS. 1B, 5A and 5B are an ideal delay profile when the frequency bandwidth is infinite.
FIG. 6 is a diagram illustrating an example where the delay profile as illustrated in FIG. 1B is actually measured. The delay profile as illustrated in FIG. 6 (on which a frequency bandwidth limit is imposed) corresponds to the delay profile as illustrated in FIG. 5A (on which no frequency bandwidth limit is imposed). As illustrated in FIG. 6, the waveforms of the delay profile blunt. Accordingly, even if it is found that the mountain-like portion including the path (1) is the region (1′) that includes the leading wave, the position of the leading wave (1) cannot be identified.
Incidentally, the delay profile is commonly measured by sampling, and the delay profile as illustrated in FIG. 6 is a delay profile measured by sampling.
As a conventional leading wave position detecting method, there is a method of detecting the position of the leading wave in terms of the position at which the sampled value (received power or the like) is maximum. The method, however, causes many erroneous detection. For example, applying the method to the case of FIG. 6, it detects the position of the path (2) as the position of the leading wave.